Electric cars aren't the only electric vehicles making their way to streets in the United States. Electric buses are beginning to find a place on the road. And there's reason to believe it isn't just a feel good initiative for cities hoping to up their green image.

Proterra, the U.S.-based bus manufacturer, says that it was able to drive one of its all-electric buses over 700 miles in one 24 hour period, apparently a record according to Proterra.

"This record offers definitive proof that all-electric Proterra buses combine all the durability and functionality of conventional buses, while significantly reducing total cost of ownership, fuel consumption and greenhouse emissions," said Garrett Mikita, CEO of Proterra, in an interview. "We are even more proud of the fact that we demonstrated this performance using a regular production bus with the same features and performance we build into all our buses."

The test run simulated commuter routes (longer routes traveled at higher speeds) and central business district routes (more stop-and-go driving). But it didn't achieve the feat without stopping to charge. The charging took place throughout the day using its proprietary "fast charging" process that allows the bus to recharge fully in the time it takes to charge a cell phone (for me that's an hour or two).

During the test, the bus traveled an average of 29 miles per hour with the HVAC system running while achieving an average fuel economy of nearly 27 miles per gallon (equivalent), or MPGe. Proterra claims that's nearly six times the average fuel economy of a diesel bus and seven times better than compressed natural gas buses. Though it certainly takes less than an hour or two to refill those buses when they near empty.

Seven hundred miles in one day is certainly impressive. But how far do buses need to travel in a day? I looked at statistics from the third-largest bus operator (by ridership) in the U.S., the Chicago Transit Authority. The CTA operates 1,781 buses (2011 statistics) and buses travel 145,832 miles in a day. Assuming all the buses are used, each bus travels roughly 81 miles in one day. Even if only half of the buses are in use each day, one bus averages just about 160 miles in a day. It's not clear if the Proterra bus could go 160 miles in a charge, but there is an all-electric bus, from Chinese competitor BYD that has already demonstrated that it can travel 200 miles on one charge.

The key for electric bus companies like Proterra and BYD will be proving to cities that electric buses can be more efficient and economical than diesel, natural gas, or hybrid buses when you factor in the costs of charging infrastructure and time needed to charge them. We're definitely getting closer to that point, if we're not already there. (Some cities are already convinced.)

In fact, Proterra's all-electric buses have already logged over 340,000 miles of revenue service in Stockton, Calif.; San Antonio, Texas; Tallahassee, Fla.; Worcester, Mass.; Reno, Nev. and Pomona, Calif. (here's the Proterra's total fleet information). And Proterra is already launching a second generation all-electric bus. So are all-electric buses ready to take over city streets? We'll wait to see if these cities increase their relatively small electric fleet (compared to their total bus fleets) and rave about it to all the other bus operators.